Patients experiencing respiratory failure often require assisted ventilation from external devices or systems to facilitate ventilation (i.e., exchange of respiratory gases) and lung expansion and thereby prevent lung collapse. One known manner for facilitating breathing in these patients is to intermittently apply negative pressure around the chest wall, creating a negative pressure in the lungs and generating inward flow of air and/or other respiratory gases into the lungs. The energy stored in the lungs and the chest wall during inspiration is utilized to move respiratory gases out of the respiratory system as the lungs and chest wall recoil during expiration. The concept of negative pressure ventilation has been known since 1670, when John Mayow first introduced a prototype of a negative pressure ventilator. The prototype consisted of a box within which a patient could sit. Attached to the box was a bladder and bellows for moving air into and out of the box. The mouth of the bladder was sealed around the patient's neck to form a closed system. Thus, movement of the bellows created a negative pressure around the patient, helping to move air into and out of the patient's lungs.
Over the years, several other ventilator models were subsequently developed based on Mayow's principle of negative pressure ventilation. In the early 1930's, the Drinker “Iron Lung” model gained wide popularity and was considered at the time to represent the state of the art for ventilation technology. By 1992, several improved portable iron lung models had been developed and manufactured. Commonly referred to as the Spencer-DHB iron lungs, these new negative pressure ventilators proved to be difficult to use due to their enormous size and weight. Prior to the 1980's, all negative pressure ventilators controlled the patient's ventilatory pattern. By the 1980's, the Emerson Company had developed a ventilator which provided assisted negative pressure ventilation. This allowed the generation of negative pressure to be coordinated with a patient's inspiratory effects, which greatly improved patient comfort and synchrony with the negative pressure ventilator. At the same time, interest in negative pressure ventilators diminished after Dominic Robert of France introduced the concept of noninvasive positive pressure ventilation via a nasal mask in the early 1980's. Robert's approach allowed assisted ventilator support with small, lightweight, portable ventilators, a significant improvement over the negative pressure ventilators available at the time.
Since Robert, noninvasive positive pressure ventilation has become increasingly popular for the provision of ventilatory support for patients with either acute or chronic ventilatory failure. The wide acceptance of noninvasive positive pressure ventilation is based in part on the many conveniences this type of ventilation offers: small size (requiring only a small dedicated floor space) simplicity of operation, and easy physical access to the patient, thereby allowing closer attention to wounds, pressure points, various catheters, intravenous injections, and bedclothes. Yet despite these benefits, noninvasive positive pressure ventilators suffer from several drawbacks. For example, noninvasive positive pressure ventilation prevents the patient from easily communicating, results in facial and oral sores, makes eating difficult, and can cause gastric distention. Although tolerated by many patients, this ventilatory approach is liked by few.
In contrast, whole body negative pressure ventilation is vastly superior in patient comfort. Whole body negative pressure ventilators allow the patient to communicate verbally and do not require sedation either to apply the ventilator itself or during its operation. Patients ventilated with these devices do not “fight” ventilatory support. Furthermore, the machine with its large capacity readily and comfortably overrides asynchronous respiratory efforts. Most importantly, negative pressure ventilation provides physiological advantages over noninvasive positive pressure ventilation. Whole body negative pressure ventilation improves the patient's cardiac output rather than reducing it, as occurs with positive pressure ventilation. During negative pressure ventilation, mean intra-thoracic pressure is decreased and venous return is facilitated. Whole body negative pressure ventilation also improves the matching of the patient's ventilation and perfusion, since gas moves into the lungs in a pattern similar to the patient's natural unassisted spontaneous breathing pattern. More importantly, as compared with positive pressure ventilation, negative pressure ventilation is better able to facilitate clearance of airway secretions, avoiding repetitive airway suctioning and bronchoscopy as well as tracheal intubation, thereby avoiding the hazards of bacterial superinfection.
Currently available negative pressure ventilation systems have been hampered by their large size and weight, lack of physical access to patients by caregivers, and limited patient comfort. The portable negative pressure ventilators presently available are not as efficient as whole body ventilator. They are difficult for the patient to attach, air leakage is very common about the seals at the neck, arms, and hips, and they cause air to be drawn across the patient's body, leading to an undesired cooling effect. These portable negative pressure ventilators also prevent patient mobility and are uncomfortable for the user. There is thus a need for a refined negative pressure ventilation system that is smaller in size, lighter in weight, easier to operate for both the caregiver and the patient, and more comfortable for the patient than currently available systems. Also desirable is a negative pressure ventilator that has more automated features to vary the breathing pattern.